Bella bought 5 packs of pencils for $21. The tax was $1. What was the price for each pack of pencils

A)
x^2 - 6x + c = - 5 + c

using a^2 - 2ab + b^2 = (a-b)^2
so
6x = 2 * x * 3
so
c = 9 

Answer
c = 9

b)
substitute c = 9
x^2 - 6x +  9 = - 5 + 9
(x - 3)^2 = 4

(x - 3)^2 = 4

c) solve for x

(x - 3)^2 = 4
(x - 3)^2 = 2^2
so
x - 3 = 2  
     x = 5

and

x - 3 = -2
     x = 1

x = 1 , 3